A scanning projection aligner typically comprises in order: a light source, a condenser, and a series of mirrors or lenses through which the image is projected until it reaches the wafer. The term "scanning" refers to the fact that the mask containing the master image and the wafer coated with photoresist are mounted on a moving carriage that is scanned past the stationary intense arc of light and the mirror or lens system. In this way, portions of the photoresist are exposed with the mask pattern. a section at a time. A subsequent development process removes some of the resist, leaving the pattern. Either the exposed portions or the unexposed portions are removed, depending on whether the resist is a positive or a negative resist, respectively.
The mask and wafer are typically about 9 inches apart, physically. However, mirrors are used to divert the light from the direct distance to a path several feet long. The image of the mask must be in focus at the plane of the photoresist in order that the image can be faithfully replicated in the resist. If the image is not in focus, the illumination will be spread out and cause loss of sharpness and critical dimension control in the resist image. To ensure that the image will be in focus at the resist, the location of the focal plane of the aligner must be routinely measured to qualify the aligner for use.
Several problems may cause the focal plane to be in the wrong place. Displacement on the order of micrometers may be enough to interfere with the success of fine line lighography. For example, if either the wafer or the mask are not perpendicular to the optical axis of the system, then they will not be parallel to each other. This will lead to a difference in optical path length between corresponding sections of mask and wafer at different parts of the wafer. The result will be that in some areas, the image may be in focus, and at others, it will be out of focus. Position misadjustment or contamination build-up on the mask (pattern side) contact plane or wafer (front side) contact plane may cause this.
The optical path length through the optics is also important. The position of mirror or lens elements may be sensitive to heating effects, mechanical adjustment, and drift due to the nature of their mounting. This can move the plane of best focus away from its target location at the resist.
Optics problems caused by misalignment of one element to another or of the illumination arc to the optics may cause an astigmatic result in some locations, in which sagittal and tangential lines are not simultaneously in focus. Finally, aberrations on mirrors or lenses may cause the focal plane location to vary.
Such problems can be detected by intentionally varying the optical path length between the mask and the wafer to locate the center of focus. Due to the nature of the mirror or lens element mounting, and the hysteresis and drift associated with any intentional focus adjustments, it is not desirable to print resolution patterns at different nominal focus settings, as is typically done on a step-and-repeat projection aligner. Rather, a special mask including a wedged, patterned section is used. The wedge places the repeated target pattern at a range of known different path lengths from the wafer.
The conventional pattern comprises a set of microscopic resolution targets that are repeated in several rows across the wedge. When the pattern is imaged onto the wafer, the resolution bars will be printed in varying degrees of focus. After the wafer is developed, the patterns are inspected through a microscope and their relative resolution is recorded by position. A poorly focused image will print resolution targets with poor dimensional control, shallow resist sidewall slopes, and resist bridging between features. A well-focused image will result in a sharp pattern for which separate mask features are distinct. The center of the best resolved patterns is at best focus and so defines the optimum optical path length. From the position of that center, the deviation from the nominal plane due to the wedge is known. In this way, best focus offsets of sagittal and tangential (or vertical and horizontal) lines at several positions along the illumination arc are determined and plotted. The relative positions of the centers of focus can form patterns characteristic of the various problems discussed above.
In practice, a mask with the center chrome patterned region offset in a plane that intersects the plane of the edges of the mask that contact the aligner's mask reference plane is used. The wedged area is mostly chrome, with microscopic resolution targets repeated in, for example, three or five rows across the mask.
The areas between the rows are not tested because of the amount of inspection that would be required. Focus in intermediate areas is assumed to be no worse than in measured locations, and most adjustable problems can be detected from first order characteristics.
When a wafer is exposed with the pattern, only those targets within the depth of focus of the resist/aligner system will be resolved in the resist. Any targets closer to, or further away from, the wafer than that will be out of focus and thus will not resolve in the resist. If a scanning aligner is correctly adjusted and the mirrors or lenses are relatively free of aberrations, then the center of focus for sagittal and tangential lines will be near the center of the mask targets. At the center, the wedge is at the same depth as the edges of the mask. The amount of deviation from the optimum focus plane can be determined knowing which targets did print in good focus and where they were on the wedge.
To interrupt the data from the conventional focus wedge wafer, approximately 100 sets of resolution bars are inspected microscopically by the operator. Resolved targets are counted from the center of the wafer. Positions with resolved targets are denoted on the data sheet by entering one type of mark, such as a plus or slash. Positions not meeting the criteria of resolution are marked with another type of mark, such as a "0". On the data sheet, the center of the resolved targets for each row and orientation is then chosen and compared with specification limits and with positions of other centers. Focus, astigmatism, parallelism, and various mirror/lens aberrations and misalignments are detected with this test.
To expose and develop a wafer in this manner will require about 5 to 10 minutes, depending on the type of equipment. An additional 4 minutes (approximately) is needed to read and record the data and to interpret and compare the data to specification limits.
A number of possible cleanings and adjustments can be performed by qualified technical people to return a machine to the desired state, where the plane of best focus is at the plane of the resist. After any adjustment, the focus wedge test is repeated to verify the correction. This can be time consuming and requires additional checking later due to the possibility of post adjustment drift.
A need remains for measuring the location of the focal plane of a scanning projection aligner across a larger area, and in a more rapid manner.